The goal of this research is to examine the relationships between social networks, suicidal ideation, non- fatal drug overdose and HIV risk behavior among adults over 18 years of age who use illicit drugs. Illicit drug users are at especially high risk for suicide, drug overdose and HIV/AIDS. Research has shown that suicidal ideation is a significant precursor for attempt and death by suicide. Starting with the work of Emile Durkheim, several studies have noted the influence of social integration and social support on risk for suicide. However, less attention has been paid to the interpersonal network structure that provides mechanisms through which integration and support operate and influence suicidal ideation. Prospective epidemiologic research will be conducted among individuals recruited from communities in the Baltimore area who inject drugs and/or use crack. The study will first compare social network structures of individuals who report suicidal ideation with those who do not at baseline. The extent to which psychological distress mediates the relationship between the change in social network structures and suicidal ideation will then be determined using structural equation modeling of baseline and 6th month data. With growing awareness of the link between mental health and behavior, there is a greater need to understand causal relationships between these constructs to inform the development of preventive interventions and enhance treatment efforts within this population. However, the current literature on suicidal risk and drug overdose is discrepant. Many speculate the association is due to some common factors. The use of baseline, 6th month, and 12th month surveys and propensity score matching is a more robust method to disentangle the relationships than conventional method of analysis. In addition, most studies on HIV and suicide has focused on how HIV infection increases risk for suicide. Whether suicidal ideation also changes the propensity of individuals to engage in behaviors that place them at risk for HIV is important but has been understudied. Using advanced statistical methods, the proposed study provides a novel approach to examine the impact of suicidal ideation on non-fatal drug overdose and HIV risk behavior. The study will also help generalize the findings to illicit drug users in the community setting. This proposed study has signficant public health potential to inform intervention strategies to reduce suicidal ideation, drug overdose and HIV transmission. The findings will help health professionals to better characterize individuals at risk for suicide and suicide attempt. In addition, this study will inform if targeting suicidal ideation may also decrease the individual's risk for drug overdose and HIV infection.